Ambient air contains water vapor. Many industrial processes require a source of air with low water vapor content. Important reasons for the removal of water vapor from air are to control the humidity of manufacturing atmospheres, to protect electrical equipment against corrosion, short circuits, and electrostatic discharges, to meet requirements for chemical processes where moisture present in air adversely affects the process, and to prevent water adsorption in pneumatic conveying. Dried air is commonly produced at the point of use. Many types of air driers and air drying processes are used.
For many years, air driers have used adsorbent compositions, sometimes called desiccants or simply adsorbents, to remove water from air streams. For economic reasons, the adsorbent compositions used in air driers are usually used more than once. Adsorbent compositions suitable for use in air driers must be capable of adsorbing and desorbing water. Many different adsorbent compositions have been used in air drying. Adsorbent compositions used in air drying processes have contained certain types of molecular sieve zeolites, silica gels, and activated aluminas. Types 4A and 13X molecular sieve zeolites have been used in adsorbent compositions for air driers. Those molecular sieve zeolites have suitable water adsorbing and desorbing characteristics for use in air driers. These 4A or 13X molecular sieve zeolites are sometimes used in combination with other adsorbents to remove water and sometimes carbon dioxide and hydrocarbons from air streams.
Air driers are generally designed with at least two adsorbent chambers or vessels. This arrangement permits production of a continuous supply of dried air. Adsorbent chambers or vessels at least partially filled with adsorbents are sometimes called "adsorbers". Typically, one chamber having "dry" adsorbent is connected with the air stream and water vapor in the air is adsorbed. While that chamber is adsorbing water, the other chamber with "wet" adsorbent is isolated from the air stream and water is removed from the adsorbent. The air drying operation is sometimes called "adsorption" or simply "drying". The adsorbent drying operation is sometimes called "regeneration" or "reactivation". Air driers are designed to cycle between the chambers so that one operates in the drying mode, while another operates in the regeneration mode.
A variety of regeneration processes are used. A common method is called "thermal" or "heat" regeneration. Thermal regeneration involves heating the adsorbent composition to a temperature at which its adsorptive capacity is reduced. At the lower equilibrium adsorption capacity, the water leaves the adsorbent surface and is removed by a stream of "purge" gas or by vacuum. The temperature to which the adsorbent composition must be heated is determined primarily by the degree to which the air must be dried and the required rate of regeneration. Other factors being equal, the dew point produced by a thermally regenerated air drier using molecular sieve zeolite adsorbent compositions will be lower as the regeneration temperature increases from 250.degree. F. to 600.degree. F. Regeneration temperatures of 300.degree. F. to 500.degree. F. are usually employed. Thermal regeneration is commonly conducted at pressures below the pressures at which the air drying operation is conducted.
Air drying is conducted at the pressures of the air stream being dried. The air stream typically enters the adsorber for drying at pressures of 30 to several thousand pounds per square inch ("psig"), although some driers operate at pressures only marginally above atmospheric. The pressure in a chamber is typically reduced for thermal regeneration. Although the pressure is generally reduced for thermal regeneration, the external heat supplied to the adsorbent composition provides the primary thermodynamic driving force to regenerate the adsorbent composition in thermally regenerated air driers.
After an adsorbent composition has been thermally regenerated, a cooling period may be used to reduce the adsorbent temperature to nearly that of the stream being processed before the vessel is repressurized. Cooling is usually regulated so as to leave the adsorbent composition at a temperature within about 50.degree. F. of the air stream to be dried. Subsequently, the pressure in the chamber is increased to the pressure of the air stream to be dried.
In sum, thermally regenerated adsorbent chambers or vessels typically operate in a cycle which includes the following steps: adsorb, depressure, heat adsorbent, cool adsorbent and repressure. The chambers of thermally regenerated air driers are generally sized to permit operation in the drying mode for periods of between about 1 to 24 hours and more typically between about 2 to 8 hours.
Certain molecular sieve zeolites are used to produce oxygen enriched air by a pressure-swing adsorption-desorption process, sometimes called "heatless" adsorption-desorption. This process operates by selective adsorption of nitrogen from air by molecular sieve zeolites at high pressure and desorption at lower pressure. Molecular sieve zeolites, such as type 5A, that exhibit a particularly strong selectivity for adsorbing nitrogen in preference to oxygen are used in this process.
This process is described generally in C. W. Skarstrom, U.S. Pat. No. 2,944,627. The adsorbent chambers of a heatless adsorption-desorption process are not regenerated by the use of external heat. A thermal regeneration step is not involved. Very short cycle times are used in heatless adsorption-desorption processes. The chambers of a heatless adsorption-desorption process are typically sized to permit operation in the adsorption mode for periods between about 10 seconds to 3 minutes.